This study investigates the safety and tolerability of losartan administered concurrently with the usual dose (10mg BID) of enalapril in patients with NYHA Functional Class III to IV heart failure treated previously with an ACE inhibitor. Additionally, to investigate the effects of the three treatment regimens of losartan administered in addition to usual dose enalapril and high dose enalapril on submaximal exercise tolerance, neurohormone activation and quality of life.